The invention describes a method for optical irradiation to develop a refractive index pattern and more particularly, a method using photo-patterning technology to develop refractive index structures in photosensitive thin film materials.
Photo-lithographic patterning typically focuses on the formation of two-dimensional patterns in a material, involving the use of multiple chemical and mechanical processing steps and material deposition steps. A positive or negative image of the desired configuration is first introduced into a material by exposing it to patterned radiation that induces a chemical change in the exposed portions of the resist. This chemical change is then exploited to develop a pattern in the material, which is then transferred into the substrate underlying the resist. Photo-patterning is used to alter the chemical resistance of photoresist material that is not retained in the finished product. The photo-patterned photoresist provides an opportunity to selectively deposit materials at precise locations over the surface of the piece or to selectively etch certain locations. Thus, refractive index structures are usually formed using dissimilar material types (such as glass versus crystalline or insulator versus semiconductor) or material compositions (varied dopant identities or concentrations) whose spatial distribution is formed via the photo-patterning of photoresist layers. Because of this use of multiple materials, multiple photo-patterning steps and chemical processing of the piece are generally required. These steps also require precise registry of the mask images used in consecutive photo-patterning steps.
Other methods for the formation of refractive index structures include electron and ion beam etching (including reactive ion etching). In these cases, spatial patterning of the refractive index is dictated by selective exposure (etching) of the parent material with a particle beam. This is followed by further thermal or chemical processing and/or deposition of dissimilar materials to provide a finished product. These energetic particle approaches require vacuum chamber technology and can be expensive. With control of the etching conditions, it is possible to build in some control of the structure cross section with depth into the material but the types of structure possible are limited and, again, the overall process in multi-step in nature, yielding a heterogeneous material structure.
Mechanical machining might also be used but the difficulties in the formation of complex shapes at a small scale tend to make this approach expensive and not amenable to large volume production.
Useful would be a method for producing changes in the refractive index values of specified volumes of a photosensitive material using a single, direct-write optical method to produce a three-dimensional refractive index structure.
According to the present invention, a method of making a refractive index structure in a photosensitive material using photo-patterning is provided. The wavelengths at which a photosensitive material exhibits a change in refractive index upon exposure to optical radiation is first determined and then a portion of the surface of the photosensitive material is optically irradiated at a wavelength at which the photosensitive material exhibits a change in refractive index using a designed illumination system to produce a three-dimensional refractive index structure. The photosensitive material is selected from compounds that can encompass inorganic, organic, and hybrid (inorganic-organic composite) materials, including oxide and non-oxide glasses such as germanosilicates, polygermanes, polysilanes, polygermane and polysilane sol-gel hybrids, and photo-active polymers such as functionalized polyesters. The illumination system can be a micro-lenslet array, a macroscopic refractive lens array, and a binary optic phase mask.
In one embodiment, a laser, such as an excimer laser, is used to optically irradiate the surface. A light coherence length less than the photosensitive material thickness is generally utilized. In general, the wavelength of the optical radiation is chosen to match an absorption band of the photosensitive material.
In another embodiment, the photosensitive material is first deposited on a substrate providing high transmission or high absorption at the wavelength used during photo-exposure, such as fused silica glass.
In another embodiment, a film is first deposited on the surface of a photosensitive material to produce a stack, with the said film selected from a reflective material and a photoresist material. This film stack is then illuminated using a designed illumination system to produce a three-dimensional refractive index structure. The optically irradiated volume can undergo either a positive or negative refractive index change.
In another embodiment, the three-dimensional refractive index structure has triangular-cross-section, low refractive index regions that traverse the thickness of the photosensitive material.